GitLab

Summary:
Use existing workaround for batched 3x3 symeig because it is faster than torch.symeig.

Added benchmark showing speedup. True = workaround.
```
Benchmark                Avg Time(μs)      Peak Time(μs) Iterations
--------------------------------------------------------------------------------
normals_True_3000            16237           17233             31
normals_True_6000            33028           33391             16
normals_False_3000        18623069        18623069              1
normals_False_6000        36535475        36535475              1
```

Should help https://github.com/facebookresearch/pytorch3d/issues/988

Reviewed By: nikhilaravi

Differential Revision: D33660585

fbshipit-source-id: d1162b277f5d61ed67e367057a61f25e03888dce